NullPointer exception while parsing JSON Map in java

Question

I'm trying to parse productId, name, and price from my below JSON in the REST API - but I'm getting null pointer exception when I try to get he name - String name = (String) obj.get("name");, may I know what I'm doing wrong here?

Any help would be really appreciated. Thanks!

JSON:

{
    "id": "af0b86eb-046c-4400-8bc4-0e26042b8f53",
    "products": [{
            "productId": "1234",
            "name": "apple",
            "price": "383939"
        }
    ]
}

Controller.java

public ResponseEntity<Object> create(@RequestBody Map<String, Object> obj) {
    Product response = myservice.create(obj);
    return new ResponseEntity<Object>(response, HttpStatus.CREATED);
}

Service.java

public Product create(Map<String, Object> obj) { 
    for (Entry<String, Object> entry : obj.entrySet()) {
        System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());
    }       

    Object [] products = (Object[]) obj.get("products");
    HashMap <String, Object> productOne = (HashMap) products[0];
    String productId = (String) productOne.get("name");
    System.out.println("productId: " +productId);

    String name = (String) obj.get("name");
    System.out.println("name: " +name);
}

Output:

Key : products Value : [{productId=1234, name=apple, price=383939}]

It seems you're getting NPE on the obj instance, make sure it's correctly wired into your service — Pankaj Gadge
– Pankaj Gadge, Commented Jan 7, 2019 at 19:49

Dzmitry Bahdanovich · Accepted Answer · 2019-01-07 19:53:11Z

5

By obj.get("name"), you try getting value by key name from a top-level of your json request, while it's a property of a second-level object

In pseudo-code (skipping casting and null-checks) it should look like this: obj.get("products")[0].get("name")

edited Jan 7, 2019 at 19:53

answered Jan 7, 2019 at 19:51

Dzmitry Bahdanovich

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2 Comments

learn groovy Over a year ago

oh..How to get second-level object :(

learn groovy Over a year ago

I tried this approach obj.get("products")[0].get("name") but Java is not liking this kind of syntax

T.Nylund · Accepted Answer · 2019-01-07 20:14:17Z

2

In Service.java you print obj inside for loop. Your print says that key is products and value is an array of objects.

So in following line obj is that top-level object and not contains “name” field.

String name = (String) obj.get("name");
System.out.println("name: " +name);

What if you call obj.get(“products”) and try to cast it into a collection? Then trying to fetch the first index from the collection. It should contain the inner object which contains name key and value.

answered Jan 7, 2019 at 20:14

T.Nylund

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6 Comments

learn groovy Over a year ago

I tried this - Map<String, Object> testobj = (Map<String, Object>) obj.get("products"); System.out.println("value is: " +testobj.get("name")); but I'm getting this exception Caused by: java.lang.ClassCastException: java.util.ArrayList incompatible with java.util.Map

T.Nylund Over a year ago

Check @NiVeR answer below.

learn groovy Over a year ago

I tried but getting same kind of exception - not sure why its not working

T.Nylund Over a year ago

Hard to reproduce the problem myself but look that exception. Looks like object cannot just cast to Map because obj.get("products") seems to return List type.

T.Nylund Over a year ago

So you should cast it into List or ArrayList. Are you using some web framework like Spring here?

|

NiVeR · Accepted Answer · 2019-01-07 20:31:33Z

1

You should reason exactly as you see the json, by depth(or level).

Initially the whole json is contained in the obj map which contains two key-value pairs:

(key = "id", value = "af0b86eb-046c-4400-8bc4-0e26042b8f53")
(key = "products", value = [{"productId": "1234", "name": "apple", "price": "383939"}])

Since you are interested in product details, the first step is two extract the array products like this:

Object [] products = obj.get("products");

Now product is an array of objects. Since you know that your objects are in turn hash maps you can cast each object to a map and access the key(s) that you want:

HashMap <String, Object> productOne = (HashMap) products[0];
String productId = (String) productOne.get("productId");
String name = (String) productOne.get("name");
..

answered Jan 7, 2019 at 20:31

NiVeR

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1 Comment

learn groovy Over a year ago

I tried this but getting this exception Caused by: java.lang.ClassCastException: java.util.ArrayList incompatible with [Ljava.lang.Object; Update the code in original post.

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